The present invention relates to a voltage controlled oscillator.
Voltage controlled oscillator (VCO) is widely being used in various fields and is particularly useful in phase locked loop (PLL). PLL is usually used as clock generator with VCO being the main component for frequency generation. Microcontrollers or microprocessors often include embed PLL-based clock generators to generate clock signals at different frequencies. VCO therefore plays an important role for frequency variable oscillations. Wider clock-frequency range can allow the more flexible operations in the microcontrollers or microprocessors and broader applications.
VCO is implemented as a series of delay cells with outputs from previous stage connected to the inputs of the next stage. The number of delay stages can be either odd or even, and depend on the design of delay cells. The oscillation frequency depends on the total delay from the delay cells and can be controlled by an external voltage. Larger delays correspond to lower the oscillation frequencies, while smaller delays correspond to higher oscillation frequencies.
A conventional cross-coupled oscillator 100, shown in FIG. 1, includes a pair of NMOS transistors 103A, 103B with their sources connected to a low voltage supply VSS (or the ground). A pair of PMOS transistors 102A, 102B is respectively connected between a positive voltage supply VDD and NMOS transistors 103A, 103B. The base nodes of the NMOS transistors 103A, 103B can respectively receive two input signals INP and INN. The base nodes of the PMOS transistors 102A, 102B are cross-coupled to the drain nodes of the NMOS transistors 103A, 103B wherein two output signals OUTN and OUTP can be provided. The cross-coupled oscillator 100 can provide full rail-to-rail oscillation signals (i.e. with voltage magnitudes reaching almost VDD to almost VSS) that are relatively insensitive to noise in VDD and VSS. The cross-coupled oscillator 100 is self-oscillating at fixed oscillation frequency without being controlled by an external signal.
Microcontrollers or microprocessors are typically required to operate from several MHz to over hundreds of MHz. This frequency large range can be difficult to achieve for conventional oscillators. There is therefore a need for an oscillator capable of operating over a wide frequency range.